Process for the electrolytic refining or lead or lead alloys containing bismuth



-J. c. DITTMER 2,507,096 PROCESS FOR THE ELECTROLYTIC REFINING OF LEAD OR LEAD ALLOYS CONTAINING BISMUTH Filed April e, 1946 May 9, 1950 JOSEPH C. DI T T MER INVENTOR.

ATTORNEY Patented May 9, 1950 PROCESS FOR THE ELECTROLYTIC REFIN- IN G OF LEAD OR LEAD ALLOYS CONTAIN ING BISMUTH Joseph C. Dittmer, St. Alhans, N. Y., assignor to National Lead Company, New York, N. Y., a corporation of New Jersey Application April 6, 1946, Serial No. 660,022

3 Claims. (01. 204-66) This invention relates to the removal of bismuth from plumbic metal which may contain, in addition to bismuth, other incidentally occurring im-. purities, e. g. silver, copper, and the like, but not including appreciable quantities of arsenic and antimony.

Prior to the invention, bismuth was removed from lead by one of two methods: (a) a chemical method, and (b) an electrolytic method. In the chemical method, the bismuth contained in the impure lead was caused to react with an alkali or alakline earth metal to form an inter-metallic compound which was removed as a dross. In the electrolytic method, the impure lead constituted the anode of a cell employing an aqueous electrolyte. As a result of the electrolysis, the lead plated out on the cathode leaving the bismuth in the anode slimes. In other words, in this method, lead was electrolyzed away from the bismuth; The present invention provides an electrolytic method by means of which the bismuth is electrolyzed away from the lead.

The principal object of the present invention is the electrolytic removal of bismuth from impure lead. This and other objects of this inven-= tion will be apparent from the following description thereof.

The figure is a front elevational view, mainly diagrammatic of a preferred type of apparatus:- for carrying out the invention.

In its broadest aspects the invention contemplates the debismuthization of impure plumbic metal by electrolysis carried out in a suitable vessel or cell, wherein the impure bismuth-containing plumbic metal in a molten state acts as the cathode; molten alkali metal hydroxide, the electrolyte; and. molten plumbic metal, the anode; whereby, upon the passage of current between the anode and cathode, the bismuth most surprisingly migrates to the molten anode.

The invention and mode of operation will be more fully appreciated by reference to the figure, molten bismuth-containing plumbic metal ID, from which the bismuth is to be removed and which constitutes the cathode metal, is contained in an ordinary hemispherical iron pot or kettle I I supported by and positioned in a suitable brick setting 12, provided with a thermostatically controlled oil burner l3 for supplying heat. The pot is electrically insulated from its setting in any satisfactory way, such as by vitrefied brick or glass blocks I4. As the metal in this pot constitutes the cathode, the pot is connected at [5 to the negative pole of the generator. An electrolyte of molten alkali metal hydroxide l6 covers a the cathode metal as well as the molten plumbic metal 11 constituting the anode.

The molten anode metal is contained in a round, fiat-bottomed, shallow iron pan or pot l8, similar in shape to a household skillet or frying pan. This pan is located directly over and as close to the top surface of the cathode metal as practical operation will permit without short circuiting. The pan is attached to, supported and rotated by a vertical hollow shaft [9, attached to which is a collar 20 riding upon a ball thrust bearing 2|, which in turn rests upon the top insulated bearing 22. The vertical hollow shaft rotates in bearings 22 which are bolted to, but electrically insulated from, a supporting steel frame 23. Electrical connection to transmit power from the positive pole of the generator to the anode metal in the pan is made in any satisfactory way such as by copper collector rings 24 attached to the rotating shaft and stationary brushes 25 making contact with the collector rings. In order to rotate the shaft and the attached anode pan, thereby slowly to stir the electrolyte, a V-belt sheave 26 is attached to a reduced diameter solid extension 21 of the shaft and is driven by a V-belt 28 from a sheave 29 on a vertical motorized speed reducer 30 mounted on and preferably electrically insulated, as by an insulating block or plate 3|, from the steel frame 23 supporting the anode pan. This steel framework may also be so constructed and equipped that the whole anode assembly can be raised and transported from one location to another by means of a traveling crane. Additional precautionary electrical insulation of the whole anode assembly may be made by placing its extremities on insulating blocks or piers 32 resting on the floor 33.

It will be understood that the foregoing de' scribes only a preferred form of unit cell. The construction of the cell may be modified as convenient and, if desired, the cells may be arranged in a cascade system, the cathode metal flowing from the highest to the lowest cell, all without departing from the scope of the invention.

The electrolysis is conducted at temperatures above the melting points of the plumbic metal electrodes and preferably at as low a temperature as is practicable while maintaining both the oathode and anode and the electrolyte in a molten condition. Accordingly, for low temperature and general economy, sodium hydroxide is preferred for use as the electrolyte since this permits operation at a temperature only slightly above the melting point of lead, say at temperatures between about 330" C. and about 350 C. Although the other alkali metal hydroxide, e. g. potassium and lithium hydroxide are operable, their use is not recommended because of their high melting points and greater cost.

The current densities and voltages at which the invention operates are not critical but may vary over wide ranges. The preferred conditions include anode current densities from about 200 to 1000, preferably about 900 amperes per square foot; cathode current densities from about 100 to about 400, preferably about 300 amperes per square foot and a voltage of about 2.5. The voltage, however, will vary according to operation conditions. The time required for the removal of the bismuth varies according to the amount of bismuth to be removed, the amperage used and the efiiciency of the electrolysis.

According to the invention, the electrolysis is continued until a desired degree or substantially complete removal of bismuth has been accomplished and this may be checked by chemical analysis of a sample of the cathode metal. A peculiar darkening change in the color of the molten alkali electrolyte seems to occur when the removal of the bismuth is substantially complete, and this phenomenon may be taken as a rough indication of completion. At this point the anode assembly is removed and the electrolyte may be pumped by a centrifugal pump into a receiving pot. The cathode metal, on account of its contamination with some alkali metal deposited during the electrolysis, may be pumped into another pot where it may be treated for the removal of its contaminating alkali metal according to the method of my U. S. Patent No. 1,976,333 with water vapor and caustic soda or this process may be applied directly in the original cathode pot without removal of the electrolyte by putting into the pot a stirrer capable of producing vertical agitation and introducing water vapor in some form.

Under the method of electrolysis as just described, the anode metal becomes covered with a soft porous crust of lead oxide saturated with molten alkali which can easily be removed from time to time during the electrolysis. If not removed, the bismuth deposits on and in this crust and not in the anode metal. In some respects, this may be considered an advantage as the deposited bismuth is more concentrated in this crust than it would be in the larger quantity of underlying anode metal. For example, at the end of the electrolysis of a batch of lead originally containing 0.1% bismuth, the metallics in the anode crust contained practically all the bismuth removed and analyzed 6.6% bismuth. If desired, the anodic metal containing bismuth which has been eliminated from the cathode may be re-utilized as an anode in the removal of bismuth from additional cathodic plumbic metal in a subsequent electrolysis according to the present invention.

The following example illustrates the practice of the present invention:

An impure lead containing 0.107% bismuth was electrolyzed in the manner above in an apparatus illustrated in the figure. Conditions of electrolysis and the results thereof are given below in tabular form.

Weight of cathode metal 200 lbs. bismuthcontaining lead Content of bismuth in cathode 0.107%

Weight of anode metal 100 lbs. lead Weight of electrolyte 50 lbs. caustic soda Current used 19'7206 amperes Voltage 2.32.5 volts.

Time of electrolysis 31 hours Average temperature 334 C.

Bismuth content of cathode .after 31 hours electrolysis 0.021% Percent bismuth removed 80+% Continued operation further reduces the bismuth and a substantially complete removal may be effected, if desired.

The electrolytic action which takes place in the practice of this invention is unique and appears to be the opposite of that which usually occurs in electro-chemical processes wherein, ordinarily, metal is dissolved at the anode and deposited at the cathode. Contrary as the principle of the present invention may be to expected procedure, it can, nevertheless, be demonstrated by checking the polarity of the electrodes. One convenient method for making such check is to immerse wires from the two electrodes in a neutral solution of sodium chloride containing a few drops of phenolphthalein which, upon passage of current through the solution, indicates the cathode wire by developing in the vicinity of the cathode wire its characteristic, alkali-produced pink to purple color. That the bismuth actually migrates from cathode to anode in the present invention is proven by chemical analyses of the cathode and anode metals before and after electrolysis.

Whatever may be the explanation of the principle of the present invention, the facts are as stated. It is possible that a more or less definite mixture of sodium bismuthide and sodium plumbide or a complex combination corresponding to the formula NaIPbyBiz may be formed, which upon electrolysis dissociates with the sodium attracted to the cathode and the bismuth depositing at the anode, possibly as an inter-metallic compound or alloy of lead and bismuth. What occurs in the present invention difiers from what takes place when an impure antimony-containing plumbic metal is electrolyzed under similar conditions, although bismuth and antimony are very similar in their chemical properties. In the course of the investigations leading to the present invention, it was found that when an antimonycontaining plumbic metal was electrolyzed, the antimony migrates from the anode to the cathode. The precise mechanism of this migration, as well as the precise mechanism of the migration of bismuth from the cathode to the anode in the practice of the present invention, is not known. It may be that with antimony a different kind of complex is formed at the anode, for instance. an alkali metal antimonite, such as NaSbO, which dissociates to yield metallic antimony, which migrates to and deposits at the cathode. Furthermore, in the electrolysis of the present invention, no indication of deposition of bismuth on the iron parts of the anode, for instance, the pot holding the anodic metal, has been observed.

It is presumed that it is for this reason impure lead containing appreciable amounts, that are over about 0.5%, of either antimony or arsenic or these combined, cannot be successfully purified according to this invention.

This application is a continuation-impart of my application Serial No. 419,558, filed November 18, 1941 (now abandoned).

The foregoing description and example has been given for illustrative purposes only and no undue limitations should be deduced therefrom.

I claim:

1. An electrolytic process for removing bismuth from bismuth-containing plumbic metal, free from appreciable amounts of antimony and arsenic, which comprises melting such plumbic metal, employing the same while molten as the cathode of an electrolytic cell, providing as an anode of said cell a pan shaped iron container containing anodic molten plumbic metal arranged so that the major current path is from the cathode to the external pan surface, and as electrolyte molten alkali metal hydroxide, and while maintaining cathode, anodic plumbic metal and electrolyte in molten condition, passing an electric current between said cathode and anode until bismuth is removed from said cathode and. transferred to said anodic plumbic metal, discontinuing the passage of current and recovering the purified plumbic cathode metal.

2. Process to claim 1 wherein the alkali metal hydroxide is sodium hydroxide.

3. Process according to claim 1 wherein the anode contains a plumbic metal containing bismuth derived from a previous electrolysis according to said claim 1.

JOSEPH C. DITTMER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

1. AN ELECTROLYTIC PROCESS FOR REMOVING BISMUTH FROM BISMUTH-CONTAINING PLUMBIC METAL, FREE FROM APPRECIABLE AMOUNTS OF ANTIMONY AND ARSENIC, WHICH COMPRISES MELTING SUCH PLUMBIC METAL, EMPLOYING THE SAME WHILE MOLTEN AS THE CATHODE OF AN ELECTROLYTIC CELL, PROVIDING AS AN ANODE OF SAID CEL A PAN SHAPED IRON CONTAINER CONTAINING ANODIC MOLTEN PLUMBIC METAL ARRANGED SO THAT THE MAJOR CURRENT PATH IS FROM THE CATHODE TO THE EXTERNAL PAN SURFACE, AND AS ELECTROLYTE MOLTEN ALKALI METAL HYDROXIDE, AND WHILE MAINTAINING CATHODE, ANODIC PLUMBIC METAL AND ELECTROLYTE IN MOLTEN CONDITION, PASSING AN ELECTRIC CURRENT BETWEEN SAID CATHODE AND ANODE UNTIL BISMUTH IS REMOVED FROM SAID CATHODE AND TRANSFERRED TO SAID ANODIC PLUMBIC METAL, DISCONTINUING THE PASSAGE OF CURRENT AND RECOVERING THE PURIFIED PLUMBIC CATHODE METAL. 